Correlation between nitrate concentration in groundwater and parameters affecting aquifer intrinsic vulnerability

This paper is the result of a study which was carried out in order to verify if the traditional methods to evaluate the intrinsic vulnerability or vulnerability related parameters, are able to clarify the problem of nitrate pollution in groundwater. In particular, the aim was to evaluate limitations and problems connected to aquifer vulnerability methods applied to nitrate contamination prevision in groundwater. The investigation was carried out by comparing NO₃ ⁻ concentrations, measured in March and November 2004 in the shallow aquifer, and the vulnerability classes, obtained by using GOD and TOT methods. Moreover, it deals with a comparison between NO₃ ⁻ concentrations and single parameters (depth to water table, land use and nitrogen input). The study area is the plain sector of Piemonte (Northern Italy), where an unconfined aquifer nitrate contamination exists. In this area the anthropogenic presence is remarkable and the input of N-fertilizers and zootechnical effluents to the soil cause a growing amount of nitrates in groundwater. This approach, used in a large area (about 10,000 km²) and in several monitoring wells (about 500), allowed to compare the efficiency of different vulnerability methods and to verify the importance of every parameter on the nitrate concentrations in the aquifer. Furthermore it allowed to obtain interesting correlations in different hydrogeological situations. Correlations between depth to water table, land use and nitrogen input to the soil with nitrate concentrations in groundwater show unclear situations: in fact these comparisons describe the phenomenon trend and highlight the maximum nitrate concentrations for each circumstance but often show wide ranges of possible nitrate concentrations. The same situation could be observed by comparing vulnerability indexes and nitrate concentrations in groundwater. These results suggest that neither single parameters nor vulnerability methods (GOD and TOT) are able to describe individually the complex phenomena affecting nitrate concentrations in soil, subsoil and groundwater. In particular, the traditional methods for vulnerability analysis do not analyze physical processes in aquifers, such as denitrification and nitrate dilution. According to a recent study in the shallow unconfined aquifer of the Piemonte plain, dilution can be considered as the main cause for nitrate attenuation in groundwater.     

Validation of an intrinsic groundwater pollution vulnerability methodology using a national nitrate database

The importance of groundwater for potable supply, and the many sources of anthropogenic contamination, has led to the development of intrinsic groundwater vulnerability mapping. An Analysis of Co-Variance and Analysis of Variance are used to validate the extensively applied UK methodology, based upon nitrate concentrations from 1,108 boreholes throughout England and Wales. These largely confirm the current aquifer and soil leaching potential classifications and demonstrate the benefits of combining soil and low permeability drift information. European legislation such as the Water Framework Directive will require more dynamic assessments of pollutant risk to groundwater. These results demonstrate that a number of improvements are required to future intrinsic groundwater vulnerability methodologies. The vertical succession of geological units must be included, so that non-aquifers can be zoned in the same way as aquifers for water supply purposes, while at the same time recognising their role in influencing the quality of groundwater in deeper aquifers. Classifications within intrinsic vulnerability methodologies should be based upon defined diagnostic properties rather than expert judgement. Finally the incorporation into groundwater vulnerability methodologies of preferential flow in relation to geological deposits, soil type and land management practices represents a significant, but important, future challenge.

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Resumen La importancia de las aguas subterráneas en el abastecimiento de agua potable, y las muchas fuentes de contaminación antropogénica, ha llevado al desarrollo del mapeo de vulnerabilidad a la contaminación intrínseca de aguas subterráneas. Se han utilizado un análisis de co-varianza y análisis de varianza para validar la metodología que se ha aplicado extensamente en UK basada en concentraciones de nitrato de 1108 pozos a través de Inglaterra y Gales. Se confirma ampliamente las clasificaciones actuales de acuíferos y potencial de lixiviación del suelo y se demuestra los beneficios de combinar la información de suelos y derrubios de baja permeabilidad. La legislación europea tal como el Marco Directivo del Agua requerirá evaluaciones más dinámicas del riesgo a la contaminación de agua subterránea. Estos resultados demuestran que se requieren varias mejoras en las metodologías futuras de vulnerabilidad intrínseca a la contaminación de agua subterránea. Tiene que incluirse la secuencia vertical de las unidades geológicas de modo que las unidades que no son acuíferos puedan zonificarse de la misma manera que los acuíferos con fines de abastecimiento de agua, mientras que al mismo tiempo se reconozca el papel que tienen en la influencia de la calidad de agua subterránea en acuíferos profundos. Las clasificaciones de las metodologías de vulnerabilidad intrínseca deberían basarse en las propiedades diagnósticas características más que en el juicio experto. Finalmente la incorporación de flujo preferencial en las metodologías de vulnerabilidad de agua subterránea en relación a las formaciones geológicas, tipo de suelo y prácticas de manejo de la tierra representa un desafío futuro no solo significativo sino que importante.

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Résumé Limportance des eaux-souterraines pour lalimentation en eau potable, et les multiples sources de contaminations anthropiques, ont conduit au développement de cartes de vulnérabilité intrinsèque des eaux souterraines aux pollutions. Une analyse de la co-variance et lanalyse de la variance sont utilisées pour valider la méthodologie extensive utilisée en UK, basée sur les concentration en nitrates de 1108 forages en Angleterre et en Ecosse. Ceci confirme les classifications courantes basées sur la nature des aquifères et le potentiel de lessivage des sols, et démontre le bénéfice que lon peut tirer à combiner linformation dérivant des sols et des faibles perméabilités. La législation européenne telle la Directive Cadre Européenne demandera des évaluations plus dynamiques des risques de pollution affectant les eaux souterraines. Ces résultats démontrent quun certain nombre daméliorations sont nécessaires aux futures méthodes de vulnérabilité intrinsèque des eaux souterraines. La succession verticale dunités géologiques doit être pris en compte, de telle manière que les parties non-aquifères puissent être zonées de la même manière que les aquifères pour lapprovisionnement en eaux potables, et reconnaître par la même occasion leur influence sur la qualité des eaux souterraines dans les aquifères profonds. Les classifications utilisées dans les méthodes de vulnérabilité intrinsèque devraient être basées sur les propriétés dun diagnostique défini plutôt que sur un jugement expert. Finalement lincorporation, dans les méthodes de vulnérabilité intrinsèque, des écoulements préférentiels en relation avec les formations géologiques, les types de sol et les pratiques daménagement du territoire représentent un challenge futur significatif et ortant.

     

Regional assessment and mapping of groundwater vulnerability to contamination

In many regions the use of groundwater for water supply is limited by the increase in contamination of aquifers. The problem of contamination requires the development of new approaches in assessing areas in which groundwater is vulnerable to contamination. A brief analysis of the present day level of methods for regional evaluation and mapping of areas in which groundwater is vulnerable is presented.     

Evaluation of an intrinsic and a specific vulnerability assessment method in comparison with groundwater salinisation and nitrate contamination levels in two agricultural regions in the south of Portugal

The applicability of two vulnerability assessment methods in evaluating the impact of agricultural activities on groundwater quality, is tested in two areas in the south of Portugal with modest results. Intensive citri- and horticulture require large amounts of fertiliser and water supplied by irrigation, which induces groundwater salinisation and contamination by nitrates. The degree of contamination varies highly within and between the study areas and is related to hydrogeological factors as well as intensity of agricultural practices. Vulnerability mapping is performed with the intrinsic DRASTIC method and the specific Susceptibility Index (SI), which is an adaptation of DRASTIC. These methods can constitute useful groundwater management tools, for instance when designating new Nitrate Vulnerable Zones as defined in the European Directive 91/676/EEC. However, in the case of DRASTIC, little correspondence exists between the most vulnerable and the most contaminated areas. This is mainly a result of underestimating the dilution capacity and overemphasising the attenuating potential of the unsaturated zone and aquifer, as both chloride and nitrate prove to be very stable contaminants. By including a parameter for land use, SI manages to produce more reliable results, although in many areas the vulnerability is overestimated.

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Resumen Se evalúa la aplicabilidad de dos métodos de estimación de vulnerabilidad en evaluar el impacto de actividades agrícolas en la calidad del agua subterránea para dos áreas en el sur de Portugal obteniendo resultados modestos. La horticultura y citricultura intensiva requiere grandes cantidades de fertilizantes y agua abastecida por riego, lo cual induce salinización de agua subterránea y contaminación por nitratos. El grado de contaminación varía fuertemente dentro y entre las áreas de estudio y se relaciona con factores hidrogeológicos así como con la intensidad de las prácticas agrícolas. El mapeo de vulnerabilidad se lleva a cabo con el método intrínsico DRASTIC y el Índice de Susceptibilidad específica (SI), el cual es una adaptación de DRASTIC. Estos métodos pueden constituir herramientas de manejo de aguas subterráneas útiles, por ejemplo al designar nuevas Zonas Vulnerables por Nitratos del modo que se definen en la Directiva Europea 91/676/EEC. Sin embargo, en el caso de DRASTIC, existen poca correspondencia entre las zonas más vulnerables y las áreas más contaminadas. Esto se debe principalmente a la subestimación de la capacidad de dilución y a al sobre énfasis del potencial de atenuación de la zona no saturada y el acuífero, ya que tanto cloruro como nitrato han probado ser contaminantes muy estables. Al incluir un parámetro del uso de la tierra, SI genera resultados más confiables, aunque en muchas áreas se sobrestima la vulnerabilidad.

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Résumé Lapplication de deux méthodes de calcul de la vulnérabilité permettant dévaluer limpact des activités agricoles sur la qualité des eaux souterraines, est testée dans deux zones du Sud du Portugal, avec des résultats modestes. La citriculture et lhorticulture intensives nécessitent de grandes quantités e fertilisants et deau souterraine pour lirrigation, ce qui induit la salinisation et la contamination des eaux souterraines par les nitrates. Le degré de contamination varie grandement à lintérieur et entre les zones détudes, en fonction des facteurs hydrogéologiques et de lintensité des pratiques agricoles. La cartographie de la vulnérabilité est mise en oeuvre via la méthodologie DRASTIC et lIndex de Susceptibilité (SI) spécifique, qui est une adaptation de la méthode DRASTIC. Ces méthodes êuvent constituer des outils de management des eaux souterraines, par exemple lors de la désignation de nouvelles zones de vulnérabilité aux Nitrates selon la n Directive Européenne 91/676/EEC. Par ailleurs dans le cas de DRASTIC, de petites correspondances existent entre les zones les plus vulnérables et les plus contaminées. Ceci est principalement le résultat dune sous-estimation de la capacité de dilution et de la sur-accentuation du potentiel datténuation de la zone non-saturée de laquifère, car et le chlore et les nitrates sont des contaminants très stables. En incluant un paramètre dutilisation des sols, SI produit des résultats plus réalistes, bien que dans de nombreuses zones la vulnérabilité soit surestimée.

     

西辽河平原(内蒙古部分)地下水固有脆弱性评价

在西辽河平原地下水资源评价的基础上,选取影响西辽河平原固有脆弱性的最主要的7个影响因子,利用GIS平台的空间分析功能,将研究区划分为14365个评价单元,运用基于DRASTIC的模糊综合评价方法对该区的地下水固有脆弱性进行了综合评价,并绘制了脆弱性分布图。评价结果表明西辽河平原(内蒙古部分)地下水脆弱性存在3个分区:大部分地区为地下水稍易污染区;南部的黄土地区为地下水稍难污染区;而西辽河和乌尔吉木伦河的中上游为地下水相对略易污染区。评价结果对该区地下水资源保护、防止地下水污染具有指导作用。     

西辽河平原(内蒙古部分)地下水固有脆弱性评价

在西辽河平原地下水资源评价的基础上,选取影响西辽河平原固有脆弱性的最主要的7个影响因子,利用GIS平台的空间分析功能,将研究区划分为14365个评价单元,运用基于DRASTIC的模糊综合评价方法对该区的地下水固有脆弱性进行了综合评价,并绘制了脆弱性分布图。评价结果表明西辽河平原(内蒙古部分)地下水脆弱性存在3个分区：大部分地区为地下水稍易污染区;南部的黄土地区为地下水稍难污染区;而西辽河和乌尔吉木伦河的中上游为地下水相对略易污染区。评价结果对该区地下水资源保护、防止地下水污染具有指导作用。     

An efficient cost-sharing program to reduce nonpoint-source contamination: theory and an application to groundwater contamination

This research evaluates the economics of cost-sharing improved irrigation technologies to reduce agricultural, nonpoint-source contamination. Irrigation and fertilization inefficiencies are modeled within a nonjoint production process to evaluate both private and public costs of technology adoption and its effect on groundwater nitrate-contamination levels. A central Nebraska application indicates that even without a current government subsidy, a farmer is economically better off switching from gravity-flow to surge-flow irrigation rather than a center-pivot system. An annual government subsidy of $22.50 (US$) per hectare per year is required over the life of a center-pivot system to make the farmer financially indifferent. However, cost-sharing center-pivot adoption improves the groundwater contamination level, while other irrigation systems result in continued deterioration of groundwater quality. Received: 3 November 1998 · Accepted: 15 February 1999     

Modelling the vulnerability of groundwater to contamination in an unconfined alluvial aquifer in Pakistan

The area of Thal Doab is located in the Indus Basin and is underlain by a thick alluvial aquifer called the Thal Doab aquifer (TDA). The TDA is undergone intense hydrological stress owing to rapid population growth and excessive groundwater use for livestock and irrigated agricultural land uses. The potential impact of these land uses on groundwater quality was assessed using a DRASTIC model in a Geographic Information System environment. Seven DRASTIC thematic maps were developed at fixed scale and then combined into a groundwater vulnerability map. The resultant vulnerability index values were grouped into four zones as low, moderate, high and very high. The study has established that 76% of the land area that is underlain by the TDA has a high to very high vulnerability to groundwater contamination mainly because of a thin soil profile, a shallow water table and the presence of soils and sediments with high hydraulic conductivity values. In addition, only 2 and 22% of the total area lie in low and moderate vulnerability zones, respectively. The outcomes of this study can be used to improve the sustainability of the groundwater resource through proper land-use management.     

Supervised committee fuzzy logic model to assess groundwater intrinsic vulnerability in multiple aquifer systems

Groundwater vulnerability modeling is an alternative approach to evaluate groundwater contamination especially in areas affected by intensive anthropogenic activities. However, the DRASTIC model as a well-known method to assess groundwater vulnerability suffers from the inherent uncertainty associated with its seven essential parameters. In this study, three different fuzzy logic (FL) models (Sugeno fuzzy logic, Mamdani fuzzy logic, and Larsen fuzzy logic) are adopted to improve the DRASTIC system to be more realistic. The vulnerability map of groundwater from multiple aquifer systems (i.e., karstic, alluvium, and complex) in Basara basin, Iraq, was created using the FL models. Validation of the FL models results using NO₃-N concentration obtained from wells and springs of the study area indicating that all of the three FL models are applicable for improving the DRASTIC model. However, each of the FL models has its own advantages for groundwater vulnerability estimation in different types of aquifer systems in the Basara basin. Therefore, this study proposes the supervised committee fuzzy logic (SCFL) as a multimodel method to combine the advantages of individual FL models. The SCFL method confirms that no water well with high NO₃-N levels would be classified as low risk and vice versa. The study suggests that this approach has provided a convenient estimation of pollution risk in the study area and therefore, a more accurate prediction of the intrinsic vulnerability to pollution in the multiple aquifer system can be achieved through SCFL method.     

Evaluation of groundwater vulnerability to pollution using DRASTIC framework and GIS

Groundwater management has a prominent role in the world especially in arid and semi-arid areas which have a shortage of water, and due to this serious problem, many researchers work on that for prevention and managing the water recourses to conserve and monitor sources. DRASTIC index can be put forward for estimating of groundwater vulnerability to such pollution. The main purpose of using the groundwater vulnerability model is to map groundwater susceptibility to pollution in different areas. However, this method has been used in various areas without modification, disregarding the effects of pollution type and characteristics. Thus, this technique must be standardized and approved for Kerman plain. Vulnerability evaluation to explain areas that are more vulnerable to contamination from anthropogenic sources has become a prominent element for land use planning and tangible resource management. This contribution aims at evaluating groundwater vulnerability by applying the DRASTIC index as well as employ sensitivity analyses to evaluate the comparative prominent of the model parameters for groundwater vulnerability in Kerman plain in the southeastern part of Iran. Moreover, the potential of vulnerability to pollution is more accurately assessed by optimizing the weights of the DRASTIC parameters with the single-parameter sensitivity analysis (SPSA). The new weights were calculated. The result of the study revealed that the DRASTIC-Sensitivity analysis exhibit more efficiently than the traditional method for a nonpoint source pollution. Observation of ultimate nitrate showed the result of DRASTIC-SPSA has more accuracy. The GIS method offers an efficient environment for carrying out assessments and greater capabilities for dealing with a huge quantity of spatial data.     

CAD-DRASTIC: chloride application density combined with DRASTIC for assessing groundwater vulnerability to road salt application

Road salt is pervasively used throughout Canada and in other cold regions during winter. For cities relying exclusively on groundwater, it is important to plan and minimize the application of salt accordingly to mitigate the adverse effects of high chloride concentrations in water supply aquifers. The use of geospatial data (road network, land use, Quaternary and bedrock geology, average annual recharge, water-table depth, soil distribution, topography) in the DRASTIC methodology provides an efficient way of distinguishing salt-vulnerable areas associated with groundwater supply wells, to aid in the implementation of appropriate management practices for road salt application in urban areas. This research presents a GIS-based methodology to accomplish a vulnerability analysis for 12 municipal water supply wells within the City of Guelph, Ontario, Canada. The chloride application density (CAD) value at each supply well is calculated and related to the measured groundwater chloride concentrations and further combined with soil media and aquifer vadose- and saturated-zone properties used in DRASTIC. This combined approach, CAD-DRASTIC, is more accurate than existing groundwater vulnerability mapping methods and can be used by municipalities and other water managers to further improve groundwater protection related to road salt application.     

Mapping composite vulnerability to groundwater arsenic contamination: an analytical framework and a case study in India

Natural Hazards - Groundwater arsenic (As) contamination affects millions of people in South Asia. In this paper, we propose a composite vulnerability framework to identify, for mitigation, the...     

Intrinsic and specific vulnerability of groundwater in central Spain: the risk of nitrate pollution

The intrinsic vulnerability of groundwater in the Comunidad de Madrid (central Spain) was evaluated using the DRASTIC and GOD indexes. Groundwater vulnerability to nitrate pollution was also assessed using the composite DRASTIC (CD) and nitrate vulnerability (NV) indexes. The utility of these methods was tested by analyzing the spatial distribution of nitrate concentrations in the different aquifers located in the study area: the Tertiary Detrital Aquifer, the Moor Limestone Aquifer, the Cretaceous Limestone Aquifer and the Quaternary Aquifer. Vulnerability maps based on these four indexes showed very similar results, identifying the Quaternary Aquifer and the lower sub-unit of the Moor Limestone Aquifer as deposits subjected to a high risk of nitrate pollution due to intensive agriculture. As far as the spatial distribution of groundwater nitrate concentrations is concerned, the NV index showed the greatest statistical significance (p?<?0.01). This new type of multiplicative model offers greater accuracy in estimations of specific vulnerability with respect to the real impact of each type of land use. The results of this study provide a basis on which to guide the designation of nitrate vulnerable zones in the Comunidad de Madrid, in line with European Union Directive 91/676/EEC.     

Combined use of 15N and 18O of nitrate and 11B to evaluate nitrate contamination in groundwater

Isotopic composition of NO₃ (δ¹⁵N_NO3 and δ¹⁸O_NO3) and B (δ¹¹B) were used to evaluate NO₃ contamination and identify geochemical processes occurring in a hydrologically complex Basin and Range valley in northern Nevada with multiple potential sources of NO₃. Combined use of these isotopes may be a useful tool in identifying NO₃ sources because NO₃ and B co-migrate in many environmental settings, their isotopes are fractionated by different environmental processes, and because wastewater and fertilizers may have distinct isotopic signatures for N and B. The principal cause of elevated NO₃ concentrations in residential parts of the study area is wastewater and not natural NO₃ or fertilizers. This is indicated by some samples with elevated NO₃ concentrations plotting along δ¹⁵N_NO3 and NO₃ mixing lines between natural NO₃ from the study area and theoretical septic-system effluent. This conclusion is supported by the presence of caffeine in one sample and the absence of samples with elevated NO₃ concentrations that fall along mixing lines between natural NO₃ and theoretical percolate below fertilized lawns. Nitrogen isotopes alone could not be used to determine NO₃ sources in several wells because denitrification blurred the original isotopic signatures. The range of δ¹¹B values in native ground water in the study area (−8.2‰ to +21.2‰) is large. The samples with the low δ¹¹B values have a geochemical signature characteristic of hydrothermal systems. Physical and chemical data suggest B is not being strongly fractionated by adsorption onto clays. δ¹¹B values from local STP effluent (−2.7‰) and wash water from a domestic washing machine (−5.7‰) were used to plot mixing lines between wastewater and native ground water. In general, wells with elevated NO₃ concentrations fell along mixing lines between wastewater and background water on plots of δ¹¹B against 1/B and Cl/B. Combined use of δ¹⁵N and δ¹¹B in the study area was generally successful in identifying contaminant sources and processes that are occurring, however, it is likely to be more successful in simpler settings with a well-characterized δ¹¹B value for background wells.     

Using soil and Quaternary geological information to assess the intrinsic groundwater vulnerability of shallow aquifers: an example from Lithuania

Lithuania, in the Baltic region of northern Europe, is heavily dependent on groundwater resources for its public water supply, with a large proportion, especially in rural areas, derived from shallow Quaternary aquifers. A national groundwater-vulnerability methodology, based upon the UK approach, has been developed on behalf of the Lithuanian Ministry of Environmental Protection as a possible basis for the future protection of shallow groundwater resources for the rural inhabitants. Some modifications to the UK methodology were required to enable archive data to be used. The four aquifer classes depicted on the final groundwater vulnerability map are based upon the assessed relative permeabilities of the uppermost Quaternary deposits. The derivation of the classification of soil-leaching potential required a reassessment of Soviet-based soil wetness and particle-size classes and a calculation of subsoil-saturated hydraulic conductivity. A preliminary validation of the final maps against available shallow groundwater samples suggests that the methodology satisfactorily predicts the intrinsic groundwater vulnerability. The final methodology, based upon its low-cost approach using archive data, is relevant to the current needs of Lithuania and can be applied in other regions of similar geology and climate. Electronic Publication     

青木关岩溶槽谷地下水水源地固有脆弱性评价

岩溶地下水脆弱性评价是基于保护岩溶含水层从而有效地管理和利用地下水提出的有效方法和手段。我国西南岩溶区大多数地区缺少应有的地下水保护带,地下水比较容易受到污染。通过采用改进的斯洛文尼亚模式,考虑四个因子:覆盖层(O)、径流特征(C)、降雨条件(P)、岩溶发育程度(K),对重庆市青木关岩溶槽谷地下水水源地的固有脆弱性进行了定量评价。结果表明:流域范围内12.6%为高脆弱性地区;43%为中等脆弱性;44.4%为低脆弱性区域。消水洞及周边100m和消水河及其两边10m,以及岩溶管道发育地区为高脆弱性区。大部分岩溶区为中等脆弱性区。砂岩地区为低脆弱性地区。评价结果与实际吻合。该评价结果为各级决策部门合理规划利用和保护地下水资源提供简单明了的科学依据。该方法对于川东岩溶槽谷地区脆弱性评价有推广价值。     

Spatial and statistical assessment of nitrate contamination in groundwater:Case of Sais Basin,Morocco

The objective of this study is to evaluate the nitrate contamination in the plioquaternary aquifer of Sais Basin based on a statistical approach. A total of 98 samples were collected in the cultivated area during the spring and autumn period of 2018. The results show that 55% and 57% of the samples in spring and autumn respectively exceed the threshold fixed by WHO(50 mg/L). However, nitrate concentrations do not show seasonal and spatial variation(p0.05). The results of the correlation matrix, principal component analysis(PCA), and hierarchical cluster analysis(HCA) suggest that nitrate pollution is related to anthropogenic source. Moreover, multiple linear regression results show that NO_3 is more positively explained in the spring period by Ca and SO_4 and negatively explained by pH and HCO_3. Regarding the autumn period, nitrate pollution is positively explained by Ca and negatively by pH. This study proposes a useful statistical platform for assessing nitrate pollution in groundwater.     

Statistical models for the assessment of nitrate contamination in urban groundwater using GIS

The purpose of this study is to develop statistical models for groundwater quality assessment in urban areas using Geographic Information Systems (GIS). To develop the models, the concentrations of nitrate (expressed as nitrogen, NO₃-N), which are different according to the type of land use, well depth and distribution of rainfall, were analyzed in the Seoul (the capital of South Korea) area. Data such as land use, location of wells and groundwater quality data for nitrate contamination were collected and a database constructed within GIS. The distribution of NO₃-N concentrations is not normal, and the results of the Mann-Whitney U-test analysis show the difference of NO₃-N concentration by well depth and by distribution of rainfall. In both the shallow and deep wells, the radius of influence is 200 m in the dry season and 250 m in the rainy season, showing the tendency to increase in the rainy season. The results of correlation and regression analysis indicate that mixed residential and business areas and cropped field areas are likely to be the major contributor of increasing NO₃-N concentration. Land uses are better correlated with NO₃-N in deep wells than in shallow wells.